Methods

Chapter 5 Methods

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

1

Objectives ✦ ✦ ✦ ✦ ✦ ✦ ✦

To declare methods, invoke methods, and pass arguments to a method (§5.2-5.4). To use method overloading and know ambiguous overloading (§5.5). To determine the scope of local variables (§5.6). To learn the concept of method abstraction (§5.7). To know how to use the methods in the Math class (§5.8). To design and implement methods using stepwise refinement (§5.10). To group classes into packages (§5.11 Optional).

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

2

Introducing Methods A method is a collection of statements that are grouped together to perform an operation. Define a method modifier method header

return value type

Invoke a method method name

formal parameters

public static int max(int num1, int num2) {

int z = max(x, y);

int result;

method body

if (num1 > num2) result = num1; else result = num2; }

parameter list

actual parameters (arguments)

return value

return result;

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

3

Introducing Methods, cont. • Method signature is the combination of the method name and the parameter list. • The variables defined in the method header are known as formal parameters. • When a method is invoked, you pass a value to the parameter. This value is referred to as actual parameter or argument.

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

4

Introducing Methods, cont. • A method may return a value. The returnValueType is the data type of the value the method returns. If the method does not return a value, the returnValueType is the keyword void. For example, the returnValueType in the main method is void.

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

5

Calling Methods Listing 5.1 Testing the max method This program demonstrates calling a method max to return the largest of the int values TestMax

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

6

animation

Calling Methods, cont. pass the value of i

public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j);

}

pass the value of j

public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2;

System.out.println( "The maximum between " + i + " and " + j + " is " + k); }

return result;

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

7

animation

Trace Method Invocation i is now 5

public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j);

public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2;

System.out.println( "The maximum between " + i + " and " + j + " is " + k); }

return result; }

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

8

animation

Trace Method Invocation j is now 2

public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j);

public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2;

System.out.println( "The maximum between " + i + " and " + j + " is " + k); }

return result; }

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

9

animation

Trace Method Invocation invoke max(i, j)

public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j);

public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2;

System.out.println( "The maximum between " + i + " and " + j + " is " + k); }

return result; }

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

10

animation

Trace Method Invocation invoke max(i, j) Pass the value of i to num1 Pass the value of j to num2

public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j);

public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2;

System.out.println( "The maximum between " + i + " and " + j + " is " + k); }

return result; }

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

11

animation

Trace Method Invocation declare variable result

public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j);

public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2;

System.out.println( "The maximum between " + i + " and " + j + " is " + k); }

return result; }

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

12

animation

Trace Method Invocation (num1 > num2) is true since num1 is 5 and num2 is 2

public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j);

public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2;

System.out.println( "The maximum between " + i + " and " + j + " is " + k); }

return result; }

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

13

animation

Trace Method Invocation result is now 5

public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j);

public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2;

System.out.println( "The maximum between " + i + " and " + j + " is " + k); }

return result; }

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

14

animation

Trace Method Invocation return result, which is 5

public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j);

public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2;

System.out.println( "The maximum between " + i + " and " + j + " is " + k); }

return result; }

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

15

animation

Trace Method Invocation return max(i, j) and assign the return value to k

public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j);

public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2;

System.out.println( "The maximum between " + i + " and " + j + " is " + k); }

return result; }

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

16

animation

Trace Method Invocation Execute the print statement

public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j);

public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2;

System.out.println( "The maximum between " + i + " and " + j + " is " + k); }

return result; }

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

17

CAUTION A return statement is required for a nonvoid method. The following method is logically correct, but it has a compilation error, because the Java compiler thinks it possible that this method does not return any value. public static int sign(int n) { if (n > 0) return 1; else if (n == 0) return 0; else if (n < 0) return –1; }

To fix this problem, delete if (n<0) in the code. Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

18

Reuse Methods from Other Classes NOTE: One of the benefits of methods is for reuse. The max method can be invoked from any class besides TestMax. If you create a new class Test, you can invoke the max method using ClassName.methodName (e.g., TestMax.max).

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

19

Call Stacks

Space required for the max method result: 5 num2: 2 num1: 5 Space required for the main method k: j: 2 i: 5

The main method is invoked.

Space required for the main method k: j: 2 i: 5

Space required for the main method k: 5 j: 2 i: 5

The max method is invoked.

The max method is finished and the return value is sent to k.

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

Stack is empty

The main method is finished.

20

animation

Trace Call Stack i is declared and initialized

public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); System.out.println( "The maximum between " + i + " and " + j + " is " + k); } public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2;

i: 5

The main method is invoked.

return result; }

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

21

animation

Trace Call Stack j is declared and initialized

public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); System.out.println( "The maximum between " + i + " and " + j + " is " + k); } public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2;

j: 2 i: 5

The main method is invoked.

return result; }

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

22

animation

Trace Call Stack Declare k

public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); System.out.println( "The maximum between " + i + " and " + j + " is " + k); } public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2;

Space required for the main method k: j: 2 i: 5

The main method is invoked.

return result; }

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

23

animation

Trace Call Stack Invoke max(i, j)

public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); System.out.println( "The maximum between " + i + " and " + j + " is " + k); } public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2;

Space required for the main method k: j: 2 i: 5

The main method is invoked.

return result; }

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

24

animation

Trace Call Stack pass the values of i and j to num1 and num2 public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); System.out.println( "The maximum between " + i + " and " + j + " is " + k);

num2: 2 num1: 5

} public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; return result; }

Space required for the main method k: j: 2 i: 5

The max method is invoked. Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

25

animation

Trace Call Stack pass the values of i and j to num1 and num2

public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); System.out.println( "The maximum between " + i + " and " + j + " is " + k);

result: num2: 2 num1: 5

} public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; return result; }

Space required for the main method k: j: 2 i: 5

The max method is invoked. Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

26

animation

Trace Call Stack (num1 > num2) is true

public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); System.out.println( "The maximum between " + i + " and " + j + " is " + k);

result: num2: 2 num1: 5

} public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; return result; }

Space required for the main method k: j: 2 i: 5

The max method is invoked. Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

27

animation

Trace Call Stack Assign num1 to result

public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); System.out.println( "The maximum between " + i + " and " + j + " is " + k); } public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; return result; }

Space required for the max method result: 5 num2: 2 num1: 5 Space required for the main method k: j: 2 i: 5

The max method is invoked. Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

28

animation

Trace Call Stack Return result and assign it to k

public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); System.out.println( "The maximum between " + i + " and " + j + " is " + k); } public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; return result; }

Space required for the max method result: 5 num2: 2 num1: 5 Space required for the main method k:5 j: 2 i: 5

The max method is invoked. Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

29

animation

Trace Call Stack Execute print statement

public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); System.out.println( "The maximum between " + i + " and " + j + " is " + k); } public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2;

Space required for the main method k:5 j: 2 i: 5

The main method is invoked.

return result; }

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

30

Passing Parameters public static void nPrintln(String message, int n) { for (int i = 0; i < n; i++) System.out.println(message); }

Suppose you invoke the method using nPrintln(“Welcome to Java”, 5);

What is the output? Suppose you invoke the method using nPrintln(“Computer Science”, 15);

What is the output?

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

31

Pass by Value Listing 5.2 Testing Pass by value This program demonstrates passing values to the methods. TestPassByValue

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

32

Pass by Value, cont. The values of num1 and num2 are passed to n1 and n2. Executing swap does not affect num1 and num2. Space required for the swap method temp: n2: 2 n1: 1 Space required for the main method num2: 2 num1: 1

The main method is invoked

Space required for the main method num2: 2 num1: 1

The swap method is invoked

Space required for the main method num2: 2 num1: 1

The swap method is finished

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

Stack is empty

The main method is finished

33

Overloading Methods Listing 5.3 Overloading the max Method public static double max(double num1, double num2) { if (num1 > num2) return num1; else return num2; } TestMethodOverloading

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

34

Ambiguous Invocation Sometimes there may be two or more possible matches for an invocation of a method, but the compiler cannot determine the most specific match. This is referred to as ambiguous invocation. Ambiguous invocation is a compilation error.

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

35

Ambiguous Invocation public class AmbiguousOverloading { public static void main(String[] args) { System.out.println(max(1, 2)); } public static double max(int num1, double num2) { if (num1 > num2) return num1; else return num2; } public static double max(double num1, int num2) { if (num1 > num2) return num1; else return num2; } } Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

36

Scope of Local Variables A local variable: a variable defined inside a method. Scope: the part of the program where the variable can be referenced. The scope of a local variable starts from its declaration and continues to the end of the block that contains the variable. A local variable must be declared before it can be used. Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

37

Scope of Local Variables, cont. You can declare a local variable with the same name multiple times in different nonnesting blocks in a method, but you cannot declare a local variable twice in nested blocks.

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

38

Scope of Local Variables, cont. A variable declared in the initial action part of a for loop header has its scope in the entire loop. But a variable declared inside a for loop body has its scope limited in the loop body from its declaration and to the end of the block that contains the variable.

The scope of i The scope of j

public static void method1() { . . for (int i = 1; i < 10; i++) { . . int j; . . . } }

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

39

Scope of Local Variables, cont. It is fine to declare i in two non-nesting blocks public static void method1() { int x = 1; int y = 1;

It is wrong to declare i in two nesting blocks public static void method2() { int i = 1; int sum = 0;

for (int i = 1; i < 10; i++) { x += i; } for (int i = 1; i < 10; i++) { y += i; }

for (int i = 1; i < 10; i++) { sum += i; } }

}

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

40

Scope of Local Variables, cont. // Fine with no errors public static void correctMethod() { int x = 1; int y = 1; // i is declared for (int i = 1; i < 10; i++) { x += i; } // i is declared again for (int i = 1; i < 10; i++) { y += i; } } Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

41

Scope of Local Variables, cont. // With no errors public static void incorrectMethod() { int x = 1; int y = 1; for (int i = 1; i < 10; i++) { int x = 0; x += i; } }

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

42

Method Abstraction You can think of the method body as a black box that contains the detailed implementation for the method. Optional arguments for Input

Optional return value

Method Signature Method body

Black Box

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

43

Benefits of Methods • Write a method once and reuse it anywhere. • Information hiding. Hide the implementation from the user. • Reduce complexity.

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

44

The Math Class ✦

Class constants: – PI –E

✦

Class methods: – Trigonometric Methods – Exponent Methods – Rounding Methods – min, max, abs, and random Methods

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

45

Trigonometric Methods ✦

sin(double a)

✦

cos(double a)

✦

tan(double a)

✦

acos(double a)

✦

asin(double a)

✦

atan(double a) Radians

Examples: Math.sin(0) returns 0.0 Math.sin(Math.PI / 6) returns 0.5 Math.sin(Math.PI / 2) returns 1.0 Math.cos(0) returns 1.0 Math.cos(Math.PI / 6) returns 0.866 Math.cos(Math.PI / 2) returns 0

toRadians(90) Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

46

Exponent Methods ✦

exp(double a) Returns e raised to the power of a.

✦

log(double a) Returns the natural logarithm of a.

✦

log10(double a) Returns the 10-based logarithm of a.

✦

pow(double a, double b) Returns a raised to the power of b.

✦

sqrt(double a) Returns the square root of a.

Examples: Math.exp(1) returns 2.71 Math.log(2.71) returns 1.0 Math.pow(2, 3) returns 8.0 Math.pow(3, 2) returns 9.0 Math.pow(3.5, 2.5) returns 22.91765 Math.sqrt(4) returns 2.0 Math.sqrt(10.5) returns 3.24

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

47

Rounding Methods ✦

double ceil(double x) x rounded up to its nearest integer. This integer is returned as a double value.

✦

double floor(double x) x is rounded down to its nearest integer. This integer is returned as a double value.

✦

double rint(double x) x is rounded to its nearest integer. If x is equally close to two integers, the even one is returned as a double.

✦

int round(float x) Return (int)Math.floor(x+0.5).

✦

long round(double x) Return (long)Math.floor(x+0.5).

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

48

Rounding Methods Examples Math.ceil(2.1) returns 3.0 Math.ceil(2.0) returns 2.0 Math.ceil(-2.0) returns –2.0 Math.ceil(-2.1) returns -2.0 Math.floor(2.1) returns 2.0 Math.floor(2.0) returns 2.0 Math.floor(-2.0) returns –2.0 Math.floor(-2.1) returns -3.0 Math.rint(2.1) returns 2.0 Math.rint(2.0) returns 2.0 Math.rint(-2.0) returns –2.0 Math.rint(-2.1) returns -2.0 Math.rint(2.5) returns 2.0 Math.rint(-2.5) returns -2.0 Math.round(2.6f) returns 3 Math.round(2.0) returns 2 Math.round(-2.0f) returns -2 Math.round(-2.6) returns -3

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49

min, max, and abs ✦

max(a, b)and min(a, b) Returns the maximum or minimum of two parameters.

✦

abs(a) Returns the absolute value of the parameter.

✦

random() Returns a random double value in the range [0.0, 1.0).

Examples: Math.max(2, 3) returns 3 Math.max(2.5, 3) returns 3.0 Math.min(2.5, 3.6) returns 2.5 Math.abs(-2) returns 2 Math.abs(-2.1) returns 2.1

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

50

The random Method Generates a random double value greater than or equal to 0.0 and less than 1.0 (0 <= Math.random() < 1.0). Examples: (int)(Math.random() * 10)

Returns a random integer between 0 and 9.

50 + (int)(Math.random() * 50)

Returns a random integer between 50 and 99.

In general, a + Math.random() * b

Returns a random number between a and a + b, excluding a + b.

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

51

JBuilder Optional

View java.lang.Math Documentation

You can view the complete documentation for the Math class online from http://java.sun.com/j2se/1.5. 2/docs/api/index.htm.

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

52

Stepwise Refinement (Optional) The concept of method abstraction can be applied to the process of developing programs. When writing a large program, you can use the “divide and conquer” strategy, also known as stepwise refinement, to decompose it into subproblems. The subproblems can be further decomposed into smaller, more manageable problems. PrintCalendar

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

53

PrintCalender Case Study Let us use the PrintCalendar example to demonstrate the stepwise refinement approach.

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

54

Design Diagram printCalendar (main)

printMonth

readInput printMonthTitle getMonthName

printMonthBody getStartDay getTotalNumOfDays getNumOfDaysInMonth

isLeapYear Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

55

Implementation: Top-Down Top-down approach is to implement one method in the structure chart at a time from the top to the bottom. Stubs can be used for the methods waiting to be implemented. A stub is a simple but incomplete version of a method. The use of stubs enables you to test invoking the method from a caller. Implement the main method first and then use a stub for the printMonth method. For example, let printMonth display the year and the month in the stub. Thus, your program may begin like this: A Skeleton for printCalendar Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

56

Implementation: Bottom-Up Bottom-up approach is to implement one method in the structure chart at a time from the bottom to the top. For each method implemented, write a test program to test it. Both top-down and bottom-up methods are fine. Both approaches implement the methods incrementally and help to isolate programming errors and makes debugging easy. Sometimes, they can be used together.

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

57

Optional

Package

There are three reasons for using packages: 1.

2. 3.

To avoid naming conflicts. When you develop reusable classes to be shared by other programmers, naming conflicts often occur. To prevent this, put your classes into packages so that they can be referenced through package names. To distribute software conveniently. Packages group related classes so that they can be easily distributed. To protect classes. Packages provide protection so that the protected members of the classes are accessible to the classes in the same package, but not to the external classes. Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

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Package-Naming Conventions Packages are hierarchical, and you can have packages within packages. For example, java.lang.Math indicates that Math is a class in the package lang and that lang is a package in the package java. Levels of nesting can be used to ensure the uniqueness of package names. Choosing a unique name is important because your package may be used on the Internet by other programs. Java designers recommend that you use your Internet domain name in reverse order as a package prefix. Since Internet domain names are unique, this prevents naming conflicts. Suppose you want to create a package named mypackage on a host machine with the Internet domain name prenhall.com. To follow the naming convention, you would name the entire package com.prenhall.mypackage. By convention, package names are all in lowercase. Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

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Package Directories Java expects one-to-one mapping of the package name and the file system directory structure. For the package named com.prenhall.mypackage, you must create a directory, as shown in the figure. In other words, a package is actually a directory that contains the bytecode of the classes.

com.prenhall.mypackage

The com directory does not have to be the root directory. In order for Java to know where your package is in the file system, you must modify the environment variable classpath so that it points to the directory in which your package resides. Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

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Setting classpath Environment The com directory does not have to be the root directory. In order for Java to know where your package is in the file system, you must modify the environment variable classpath so that it points to the directory in which your package resides. Suppose the com directory is under c:\book. The following line adds c:\book into the classpath: classpath=.;c:\book;

The period (.) indicating the current directory is always in classpath. The directory c:\book is in classpath so that you can use the package com.prenhall.mypackage in the program.

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

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Putting Classes into Packages Every class in Java belongs to a package. The class is added to the package when it is compiled. All the classes that you have used so far in this book were placed in the current directory (a default package) when the Java source programs were compiled. To put a class in a specific package, you need to add the following line as the first noncomment and nonblank statement in the program: package packagename;

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Listing 5.8 Putting Classes into Packages Problem This example creates a class named Format and places it in the package com.prenhall.mypackage. The Format class contains the format(number, numOfDecimalDigits) method that returns a new number with the specified number of digits after the decimal point. For example, format(10.3422345, 2) returns 10.34, and format(-0.343434, 3) returns –0.343. Solution 1. Create Format.java as follows and save it into c:\book\com\prenhall\mypackage. // Format.java: Format number. package com.prenhall.mypackage; public class Format { public static double format( double number, int numOfDecimalDigits) { return Math.round(number * Math.pow(10, numOfDecimalDigits)) / Math.pow(10, numOfDecimalDigits); } }

2. Compile Format.java. Make sure Format.class is in c:\book\com\prenhall\mypackage.

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

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Using Classes from Packages There are two ways to use classes from a package. • One way is to use the fully qualified name of the class. For example, the fully qualified name for JOptionPane is javax.swing.JOptionPane. For Format in the preceding example, it is com.prenhall.mypackage.Format. This is convenient if the class is used a few times in the program. • The other way is to use the import statement. For example, to import all the classes in the javax.swing package, you can use import javax.swing.*; An import that uses a * is called an import on demand declaration. You can also import a specific class. For example, this statement imports javax.swing.JOptionPane: import javax.swing.JOptionPane; The information for the classes in an imported package is not read in at compile time or runtime unless the class is used in the program. The import statement simply tells the compiler where to locate the classes. Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

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Listing 5.9 Using Packages Problem This example shows a program that uses the Format class in the com.prenhall.mypackage.mypackage package. Solution 1. Create TestFormatClass.java as follows and save it into c:\book. The following code gives the solution to the problem. // TestFormatClass.java: Demonstrate using the Format class import com.prenhall.mypackage.Format; public class TestFormatClass { /** Main method */ public static void main(String[] args) { System.out.println(Format.format(10.3422345, 2)); System.out.println(Format.format(-0.343434, 3)); } } Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved. 0-13-222158-6

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